Video shows the "unparalleled" find among the ruins of a monastic retreat.

Philip de Jersey really doesn't know why medieval porpoise bones were found in a grave that looks like it was dug for a human being.

"I've never seen anything that parallels it," he said.

De Jersey is the state archaeologist for the Guernsey Museum and Galleries. For the past three weeks, the museum's field archaeology crew has been conducting excavations at the site of a Medieval monastic retreat at Chapelle Dom Hue in the southwestern portion of Guernsey, an island that sits between England and France.

Beginning on August 29, de Jersey began posting daily video updates of his finds from the field. At first the dig yielded little more than sediment.

Then came day 11.

"In 35 years of digging, it's one of the strangest and most bizarre things I've ever come across," de Jersey said in the video he posted on September 15.

Video shows a grave cut into the still-moist earth. In it rests the centuries-old skull of a porpoise, with what appears to be shoulder bones flanking its sides. The skull has two, roughly one-inch holes in the top of its head, which de Jersey noted were likely drilled by hungry rats, above two small naval cavities.

Two primary theories have emerged to explain this unusual find.

The first is that the burial had intentional religious significance.

The site of Chapelle Dom Hue contains the ruins of what excavators believe is a religious retreat, or hermitage, which de Jersey said dates back to the 14th century. Monks may have once lived at and visited the small retreat. The region's religious overtones have thus led some experts to speculate that the porpoise grave was some sort of ceremonial burial.

The second theory for why porpoise bones were buried, seemingly intentionally, is that the monks were planning to eat it (or had already consumed parts of it). In the medieval era, porpoises were sometimes eaten as a delicacy. It's plausible that the porpoise was trapped in the natural pools of water that collect at the island's edges during high tide. Monks would have been able to easily catch the porpoise after tide waters receded.